1. Technical Field
The present invention relates to a information recording medium (hereinafter optionally referred to as “a recording medium”) for recording information thereon at a higher speed and at a higher density and reproducing the information therefrom, and to a process for producing the same.
2. Description of Related Art
The present inventors have developed 4.7 GB/DVD-RAM and a single-layer single-sided 25 GB (1-fold speed)/Blu-ray disc, as disc-shaped phase-change information recording media of a large capacity (hereinafter optionally referred to as “optical disc”) on and from which information is recorded and reproduced with a laser beam. These recording media can be used as data files or image files. The inventors have further developed a double-layer single-sided 50 GB (1-fold time speed)/Blu-ray disc (having two information layers at its one side) in order to increase the recording capacity of the optical disc. These discs already have been commercialized.
As the recording modes of these DVD-RAM and Blu-ray discs, phase-change recording modes are employed. This recording mode takes an advantage of the property of a recording layer's irreversible change in its state between an amorphous phase and a crystal phase (or between a crystal phase and another crystal phase having a different structure) when being irradiated with a laser beam. This is described in detail. Information is recorded by irradiating a recording layer with a laser beam to cause change of at least one of its refractive index and extinction coefficient, so that a recording mark is formed thereon. Once the recording mark has been formed, light which is emitted and transmits or reflects on the recording layer differs in its amplitude between the recording mark and other surrounding portion of the recording layer. Information (or a signal) is reproduced by detecting such a difference.
It is generally known that no information is recorded on a recording layer when the material of the recording layer is in a crystalline state. A signal is recorded on the recording layer when the recording layer is put in an amorphous state after the material of the recording layer is melted by exposure to a laser beam and is then quenched to be amorphous. The deletion of the signal is done by exposing the recording layer to a laser beam with power lower than that for the recording, and putting the recording layer in the crystalline state.
Generally, a phase-change type optical disc includes a dielectric layer, a recording layer and a reflective layer, which are formed in this order on a substrate. As an example of the structure of such a disc, a first dielectric layer, a recording layer, a second dielectric layer and a reflective layer are deposited in this order on a substrate. Hereinafter, the functions of the respective layers are described. The dielectric layer includes a dielectric material and has a function to protect the recording layer from an external mechanical damage, a function to utilize an interference effect by multiple reflection to emphasize an optical change, a function to shut out an influence from an external air to thereby prevent the chemical change of the recording layer, and a function to reduce the surface roughening of the substrate and the thermal damage of the recording layer due to the repeated recording of signals. The dielectric layer is therefore sometimes called a protective layer.
When the dielectric layer consists of two layers, one layer of the dielectric layer in contact with a recording layer (this layer being also called an interface layer) is able to vary the state-changing speed of the recording layer between a crystalline state and an amorphous state, by appropriately selecting the composition of this dielectric layer. Therefore, the dielectric layer in contact with the recording layer also has an important function to control the crystallization speed of the recording layer.
As described above, the recording layer absorbs a laser beam and changes its own phase. Information is mainly recorded on this layer. When information is recorded or deleted, the reflective layer functions to absorb heat from the recording layer which has absorbed the laser beam and has a higher temperature, and to release the heat.
The properties of each of the layers change depending on not only the composition of its material but also its thickness. That is, layers which are common in the compositions of their materials and which differ in thickness from each other show different properties. For example, when the thickness of the reflective layer is increased, it becomes possible to efficiently release heat which the recording layer has absorbed when information has been recorded on the recording layer. As a result, an amorphous portion is easily formed in the recording layer so that the quality of the signal is improved.
An information recording medium having multiple layers on its one side, typified by a double-layer single-sided optical disc such as the above-described double-layer single-sided Blu-ray disc, has a configuration wherein, as shown in FIG. 2, a first information layer 21, a second information layer 22, a third information layer 23, . . . , and a n-th information layer n are deposited in this order on a substrate 20. These information layers are optically separated from one another by transparent optically separating layers formed of a UV curable resin; and a cover layer 28 (or a light-transmitting layer) formed of a UV curable resin is provided on the n-th information layer. Recording or reproducing data on or from each of the information layers is made by causing a laser beam 29 to come into the disc from the side of the cover layer 28.
One of the properties required for the multi-layer single-sided recording medium is that the information layers near the laser beam-incoming side should have higher transmittance. For example, in case of a double-layer single-sided recording medium (i.e. a recording medium having two information layers on its one side), information is recorded on or reproduced from one of the information layers on the rear side (or distant from a laser beam (or a laser beam source)) (this layer being referred to as “a first information layer”) by using a laser beam which transmits the other information layer near the laser beam-incoming side (this layer being referred to as “a second information layer”). Therefore, laser power needed to record the information on the first information layer is represented by a value calculated as follows: laser power needed to record the information on a recording medium having only the first information layer is divided by the transmittance of the second information layer. In other words, the medium having two information layers requires more laser power to record or reproduce information. Further, in the medium having two information layers, the second information layer is needed to have a higher transmittance (for example, 50%), while the first information layer is needed to have a higher reflectance.
The present inventors studied the following technique: that is, in an information layer which has at least a recording layer and a reflective layer in this order from a laser beam-incoming side, a transmittance-adjusting layer formed of a dielectric material is provided in contact with one side of the reflective layer which is opposite the laser beam-incoming side of the reflective layer. Further, the inventors optimized the refractive indexes and the extinction coefficients of the transmittance-adjusting layer and the reflective layer. As a result, it becomes possible to increase the transmittance of the first information layer. Further, there is an approach to decrease the thickness of laser beam-absorbing layers (i.e. the recording layer and the reflective layer) so as to increase the transmittance of the first information layer.
Herein, examples of the recording media each having two information layers on its one side reported so far are described. JP-A-2001-266402 discloses a double-layer single-sided optical disc, wherein the first information layer includes layers of AlCr/ZnS—SiO2/GeSbTe/ZnS—SiO2 deposited in this order from the side near to a substrate on which the layers are formed (i.e. a substrate attached to a film-forming apparatus), and wherein the second information layer includes layers of ZnS—SiO2/InSbTe/ZnS—SiO2. In this regard, the slash “/” means the lamination, and the dash “-” means the mixing of the substances. In the recording medium disclosed in this Publication, the two information layers have recording layers of which the compositions are different from each other.
JP-A-2002-298433 discloses a double-layer single-sided optical recording medium, wherein the first information layer includes SiC (a transparent heat-releasing layer)/Au (a reflective layer)/ZnS—SiO2 (a dielectric layer)/GeN (a crystallization-accelerating layer)/Ge5Sb76Te19 (a recording layer)/GeN (a crystallization-accelerating layer)/ZnS—SiO2 (a dielectric layer), and wherein the second information layer includes an Ag alloy (a reflective layer)/ZnS—SiO2 (a dielectric layer)/GeSiN (a crystallization-accelerating layer)/Ge2Sb2Te5 (a recording layer)/GeSiN(a crystallization-accelerating layer)/ZnS—SiO2 (a dielectric layer). In the recording medium disclosed in this Publication, the reflective layer, the recording layer and the crystallization-accelerating layer in contact with the recording layer in the first information layer are different in their compositions from those in the second information layer.
International Symposium Optical Memory (ISOM 2000), Technical Digest, pp 16-17, discloses one model of double-layer single-sided optical disc developed by the present applicant (i.e. the assignee in the U.S.A). In this optical disc having two information layers on its one side, the first information layer includes an Al alloy (a reflective layer)/ZnS—SiO2 (a protective layer)/GeN (an interface layer)/GeSbTe (a recording layer)/GeN (an interface layer)/ZnS—SiO2 (a protective layer); and the second information layer includes an Ag alloy (a reflective layer)/ZnS—SiO2 (a protective layer)/GeN (an interface layer)/GeSbTeSn (a recording layer)/GeN (an interface layer)/ZnS—SiO2 (an interface layer). In this optical disc, the recording layer and the reflective layer in the first information layer are different in their compositions from those in the second information layer.
In any of the double-layer single-sided recording media disclosed in the above Publications, one layer or a plural number of layers, out of the layers constituting one information layer, is/are different in the composition(s) from the corresponding layer(s) in the other information layer. This is because the properties required for the second information layer are different from the properties required for the first information layer, as mentioned above.
JP-A-2005-122872 discloses, in Example 1, a double-layer phase-change type information recording medium, wherein the first information layer includes Al—Ti (a reflective layer)/ZnS—SiO2 (a protective layer)/Ge5Ag1In2Sb70Te22 (a recording layer)/ZnS—SiO2 (a protective layer); and the second information layer comprises In2O3—ZnO (a heat diffusing layer)/Ag—Zn—Al (a reflective layer)/ZnS—SiO2 (a protective layer)/Ge5Ag1In2Sb70Te22 (a recording layer)/ZnS—SiO2 (a protective layer)/In2O3—ZnO (a heat-diffusing layer). In the recording medium disclosed in this Publication, the recording layer in the first information layer is the same as that in the second information layer, and each of the recording layers in both the information layers is laid between the ZnS—SiO2 layers. In this disc, the compositions of the reflective layers in the first information layer and the second information layer are different from each other, while the compositions of the recording layer and the protective layers in the first information layer are the same as those in the second information layer. In this point, this disc differs from the media disclosed in the former three Publications.